Most present-day modem receivers compatible with the Bell Telephone 201 standards are implemented with LSI circuits or discrete digital logic elements, in combination with front-end filtering using analog components. The LSI circuits may be made for the specific modem design to provide a stable implementation and low cost in large quantities. However, custom LSI circuits inhibit design flexibility and have high initial costs. Modifications in modem design for system refinements and changes to accommodate specific modem applications are difficult, if not impossible, to make without completely redesigning the LSI circuits. Moreover, the associated analog components used in the modem receiver are susceptible to analog component tolerance and stability problems.
In some modem designs, the signal is digitized, and the filtering and processing functions are performed by specially designed circuits and systems optimized for signal processing using general digital signal processing techniques. The implementation of modem receivers in these systems is typically little more than a direct transformation and adaptation of general signal processing theory, which places a burden on the signal processing hardware to produce real-time performance. As a result, previous modem receivers having digital signal procesors require large or high-speed and therefore costly data processing structures.